Colitis is an inflammatory disease of the intestine which has traditionally been attributed to Th1-type CD4+ T cell responses. We previously established a model in which colitis can be triggered in IL-10 deficient mice or mice treated with anti-IL-10R by infection with the common intestinal bacteria Helicobacter hepaticus (Hh). Similar tissue inflammatory effects could be seen in Hh infected T cell deficient RAg mice that had received CD4+ T cells. Disease in these models was previously shown to require expression of IFN-gamma and IL-12p40, a protein that is a subunit of both IL-12 and IL-23, a cytokine which has been shown in recent years to promote autoimmune inflammation. By studying mice deficient in either p35 (the chain which pairs with p40 to make IL-12) or p19 (the chain which pairs with p40 to make IL-23) we established that IL-23, not IL-12 is the critical cytokine in Hh induced colitis. IL-23 is thought to promote inflammation by triggering the cytokine IL-17 from CD4+T cells (Th17 cells)and indeed high levels of IL-17 were detected in colitic tissues. Neverthless, IFN-gamma was also detected and shown to play a major role in the induction of disease in the Hh model. Thus, these studies support a dual contribution of both IL-17 and IFN-gamma in inflammatory bowel disease and indicate that IL-23 not IL-12 should be targeted in anti-cytokine therapeutic approachs to treating this important chronic ailment.[unreadable] [unreadable] An ongoing interest of our group concerns the mechanisms by which the innate immune system senses and responds to mycobacterial infection. Our previous studies along with those of other groups have established the involvement of host Toll-like receptors (TLR) and in particular TLR2 and TLR9 working in synergy in this process. Dectin-1 is a fungal pattern recognition receptor that binds to beta-glucans and triggers cytokine production by facilitating interaction with TLR2 or by directly activating spleen tyrosine kinase (Syk). To assess the possible role of Dectin-1 in the innate response to mycobacteria, we used an in vitro system in which IL-12p40 production is measured in splenic dendritic cells (SpDC) following exposure to live Mycobacterium tuberculosis bacilli. Treatment of SpDC with laminarin or glucan phosphate, two molecules known to block Dectin-1-dependent activity, led to a reduction in M. tuberculosis-induced IL-12p40 as well as IL-12p70 production. Moreover, SpDC from Dectin-1(-/-) chimeric mice displayed reduced IL-12p40 production in response to mycobacteria when compared with Dectin-sufficient DC. Laminarin treatment also inhibited mycobacterial-induced IL-12p40 production in DC from TLR2(-/-) mice, arguing that Dectin-1 functions independently of TLR2 signaling in this system. Importantly, a Dectin-1 fusion protein was found to directly bind to live mycobacteria in a laminarin-inhibitable manner indicating the presence of ligands for the receptor in the bacterium and laminarin pretreatment resulted in reduced association of mycobacteria to SpDC. In additional experiments, mycobacterial stimulation was shown to be associated with increased phosphorylation of Syk and this response was inhibited by laminarin. Furthermore, pharmacologic inhibition of Syk reduced the M. tuberculosis-induced IL-12p40 response. Together, these findings support a role for Dectin-1 in promoting M. tuberculosis-induced IL-12p40 production by DC in which the receptor augments bacterial-host cell interaction and enhances the subsequent cytokine response through an unknown mechanism involving Syk signaling.